I have a good relationship with my outlaws. The new wife he left me for doesn't approve. She's welcome to him. She was the moment he told me about his affair. She's not welcome to rewrite my existence or relationships with my wider family.
We are linked through the blood of their DGC. I've known them all my adult life. My FIL for longer, he worked with my dad all my life.
I don't understand why my divorce means I should make adjustments to my relationships with anyone but the man I divorced. I'm not local so event attendance isn't an issue but they come to see me when ever they go to their son and his wifes house. I also have contact with my BIL who i've always got on very well with.
I have contact because they're people I've shared so much of my life with. I care for them, they for me. Its absolutely nothing to do with spiting my XH's wife.
I don't understand why she is getting so much under your skin. I read it as a mass insecurity and possibly even jealousy on your part. You need to find a coping strategy because if this has been going on fifteen years its no doubt going to go on indefinitely.
Right or wrong fair or not, it is what it is.
Look to the strengths. He chose and chooses to be with you. From the little I interpret, you too are welcomed. She is not outwardly antagonistic. The gatherings sound big so you are not on top of each other.
If it wasn't for the history of her being first wife and she was just a relative you didn't see eye to eye with how would you handle it?